Circuits that include transistors used as controllable switches or variable resistors come in various topologies including H-Bridge topologies, half H-bridge topologies and tuned amplifier topologies. Further, circuits operating as Radio Frequency (RF) power amplifiers have traditionally been implemented in a tuned amplifier topology in which input and output networks of the power amplifiers effectively resonate at a desired operating frequency. Resonant frequency operation allows parasitic reactance, such a shunt capacitance and series inductance, to be absorbed into the networks, thereby maximizing the energy transfer from the driving source to the amplifier and from the amplifier to the load. Optimal performance of such tuned amplifiers is achievable over a limited operating bandwidth, but more importantly, over a narrow range of power levels. As a result, modulation formats which have large peak-to-average ratios typically exhibit poor efficiency when amplified by tuned amplifier topologies.
Recent advances in high frequency semiconductors and digital signal processing software and hardware suggest that efficient RF power amplifiers for modulation formats with large peak-to-average ratios may be realized using non-resonant amplifier topologies which are driven by switch-mode waveforms. The H-Bridge is one suitable topology that is commonly used in motor control applications and has been adapted for use in digital audio amplifiers. The H-Bridge topology includes a left sub-circuit and a right sub-circuit, each of the sub-circuits includes an upper transistor and a lower transistor and a load is connected across the sub-circuits at nodes between the upper and lower transistors.
In operation, complementary transistors, comprising an upper transistor in one sub-circuit (limb) and a lower transistor in the opposite sub-circuit (limb), are alternately turned on causing current to flow through the load from left to right and then right to left. In low frequency applications, the upper transistors are typically P-channel enhancement-mode Field Effect Transistors (FETs), while N-channel enhancement-mode FETs are used for the lower transistors. The P-channel devices are turned on using a drive waveform which pulls their gate voltage toward ground. Conversely, the N-channel devices are turned on when their gate voltage is raised above ground. This complementary symmetry allows the H-Bridge topology to be driven by a simple ground-referenced circuit design.
The H-Bridge topology when used as a power amplifier in high frequency radio applications can have limitations when using both P-channel enhancement-mode Field Effect Transistors (FETs) and N-channel enhancement-mode FETs. This is because the poorer mobility of holes (in P-channel devices) in comparison to electrons (in N-channel devices) results in the gain-bandwidth product (Fτ) of P-channel devices being typically one-half to two-thirds that of N-channel devices. In addition, the lower transconductance of P-channel devices requires fabrication of a physically larger device structure in order to match the current capability of N-channel devices. The larger device increases shunt capacitance, further limiting high frequency performance.
An H-Bridge topology may be constructed using all N-channel devices to improve high frequency operation. However, such a configuration presents a challenging driver design problem as the source voltage of the upper transistors are not fixed. As a result, a relatively large gate voltage swing (referenced to ground) is required by the upper transistors to turn them from an on state to an off state. Such large gate to source voltage swings require significant drive power in order for transistors to change from a conductive state to a non-conductive state and vice versa especially in RF applications. Thus, although the all N-channel H-Bridge topologies are able to operate at high frequencies as power amplifiers, they require relatively large gate voltage swings to control (switch) the upper transistors. A means to provide a small, ground-referenced drive signal to the upper transistors is therefore critical to efficient operation of such topologies.
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